The causative factors for cytodifferentiation of cell types of the pars distalis are poorly understood. Our studies examine the influence of a) the homologous mesenchyme associated with Rathke's pouch, and b) estrogen, on normal pituitary cytodifferentiation and the mechanisms by which these same determinants may also influence prolactinoma formation. Additional studies are directed at the immunolocalization of pituitary estrogen receptor during normal ontogeny and during tumorigenesis. The results of these studies will therefore elucidate fundamental tissue interactions governing pituitary cytodifferentiation and their possible ramifications on tumorigenesis. Our experiments will test 4 hypotheses: 1. Direct interactions between Rathke's pouch (RP) homologous mesenchyme and RP epithelium serve as a positive stimulus for initial development of somatotrophs and gonadotrophs, but inhibit initial development of mammotrophs. Experiments will evaluate cytodifferentiation of specific cell types by electron microscopy (EM) and immunocytochemistry (ICC) in grafts derived from transplanting RP, with and without its homologous mesenchyme, either beneath the kidney capsule of adult hosts or via implantation within Nucleopore filter chambers. 2. A mesenchymal response i.e., disrupted epithelial-mesenchymal integrity and atypical vessels, is characteristic of the earliest stages of prolactinoma formation in estrogen sensitive individuals. Experiments are conducted in which the structural integrity of the epithelial-mesenchymal interface is analyzed (EM and ICC) in RP-kidney capsule grafts in hosts stimulated by estrogen, and in estrogen stimulated Fisher 344 rats (highly estrogen-sensitive) and Wistar/Lewis rats (not estrogen-sensitive). 3. Initial development of mammotrophs, and possibly other pituitary cell types, is estrogen-dependent. The presence or absence of mammotrophs will be evaluated (EM and ICC) in RP grafts in endocrinectomized hosts i.e., estrogen deficient. 4. Immunolocalization patterns of estrogen receptors during restricted stages of normal pituitary ontogeny i.e., tumor susceptible stages, are also characteristic of the earliest stages of prolactinoma formation in estrogen-sensitive adults, and will correlate with a mesenchymal response. ICC will be performed using monoclonal antibody against estrogen receptor a) at all stages of fetal development, b) in estrogen-stimulated RP grafts, and c) in estrogen-stimulated adult Fisher 344 and Wistar/Lewis rats.